In eukaryotes, protein synthesis (translation) occurs in a complex process in which messenger RNA (mRNA) carrying amino acid sequence information encoded in its nucleotide sequence interacts with ribosomes and a variety of cofactors and enzymes. Among the critical interactions are those which occur in the initial steps of mRNA recognition during initiation of translation
Synthesis of mRNA occurs in the nucleus of the eukaryotic cell. Translation occurs in the cytoplasm. RNA sythesized in the nucleus is subject to modifications, generally termed processing reactions. These include capping, intron splicing and polyadenylation. Of importance herein is the processing step known as capping. Capping is the addition, at the 5' end of mRNA, of 7-methyl guanine, (m.sup.7 G) joined by an unusual 5'--5' diphosphate bridge to the 5' terminal ribonucleotide of mRNA. The capping reaction occurs naturally in the cell nucleus during mRNA synthesis. Capping can also be carried out in vitro in an enzyme-catalyzed reaction. Commercially available kits can be obtained, for example, from Life Technologies, Inc., Gaithersburg, Md.
The initiation of translation in the cytoplasm requires specific binding of proteins termed initiation factors. An important initiation factor in mammalian cells is the eukaryotic Initiation Factor--4E (eIF-4E) which binds to capped RNA (m.sup.7 G-RNA). Translation is regulated in vivo by factors and conditions which affect the binding of eIF-4E to m.sup.7 G-RNA, including proteins that bind to eIF-4E (4E binding proteins). For example, at least one 4E binding protein designated 4E-BP-1 acts to prevent the binding of eIF-4E to m.sup.7 G-RNA. 4E-BP-1, also known as PHAS-1, can undergo phosphorylation which is induced by insulin or other growth factors. The insulin-induced phosphorylation of 4E-BP-1 releases the bound eIF-4E which is now available to bind m.sup.7 G-RNA. This process may account for the rapid stimulation of protein synthesis in muscle tissue induced by insulin. Another eIF-4E binding protein is p220, also known as eIF-4F, a protein that binds with eIF-4E as part of a functional complex which interacts with mRNA to positively regulate translation.
The sequence of DNA encoding human eIF-4E has been determined [Reychlik, W. et al. (1987) Proc. Natl. Acad. USA 84: 945-949]. Yeast eIF-4E and a fusion protein of mouse eIF-4E have been expressed in E. coli [Edery, I., et al. (1988) Gene 74:517-525; Edery, I., et al. (1995) Mol. Cell. Biol. 15: 3363-3371]. Haas, D. W. et al. (1991) Arch. Biochem. Biophys. 284:84-89 reported purification of native eIF-4E from erythrocytes. Stern, B. D. et al. (1993) reported isolation of recombinant eIF-4E using denaturing concentrations of urea. However, expression and purification of recombinant human eIF from the soluble fraction without a denaturation step was not described before.
Transfection using RNA, has not been widely reported. The primary difficulty is the susceptibility of RNA to RNAses and the lack of RNA restriction enzymes and ligases that has prevented in vitro recombination of RNA segments. Nevertheless, transfection with RNA has several advantages over transfection with DNA. Transfection by RNA does not normally lead to genetic alteration of host cells. Instead, a transient expression of the protein encoded by the transfecting RNA is observed. There are circumstances where such transient expression is preferable. For example, RNA transfected cells can transiently express an antigen in an individual to be immunized. Garrity, R. R., et al. (1996) (Abstr. 1996 Meeting on Molecular Approaches to the Control of Infectious Diseases, Cold Spring Harbor Laboratory, Cold Spring Harbor, N.Y., Sept. 9-13, 1996) reported that antibodies to gp 120 and gp 160 of HIV-1 were detectable in guinea pigs that had been injected intramuscularly with naked m.sup.7 G-RNA encoding the respective antigens. Titres were low and the antibodies did not neutralize homologous virus. Since DNA transfection leads to chromosomal integration of extraneous DNA and long-lived expression of its encoded protein, unpredictable and deleterious effects may occur in the host. Transient expression resulting from RNA transfection can avoid these concerns. The problems to be overcome with RNA transfections include extremely low transfection efficiency and short intracellular lifetime of transfected RNA.
eIF-4E has recently been shown to play a direct role in maintaining the phenotype of breast cancer cells. The levels of eIF-4E in biopsies of breast cancer and breast cancer cell lines are increased (3-30 fold; mean of 10.5.+-.0.9) as compared to benign fibroadenomas of breast tissue and control cells [Kerekatte, V. et al. (1995) Int J Cancer 64(1):27-31; Anthony, B. et al. (1996)Int J Cancer65:858-863]. Immunohistochemical studies showed that the cells expressing high levels of eIF-4E are indeed cancer cells and not stromal cells. In addition, evidence indicates that high levels of expression of eIF-4E correlate with a poor clinical outcome in breast cancer [Li, B. D. L. et al. (1997) Cancer 79(12):2385-2390]. A direct role for eIF-4E in breast cancer is evidenced by studies demonstrating that mammary carcinoma cells (MDA-435) exhibiting a 50% decrease in eIF-4E expression, due to stable transformation with an antisense construct, have a markedly reduced ability to produce tumors in nude mice. In addition, the down-regulation of eIF-4E expression in these cells results in relatively avascular tumors compared to control cells [Nathan, et al. 1997].
The cocrystal structure of mouse eIF-4E bound to m.sup.7 GDP [Marcotrigiano J. et al. (1997) Cell 89:951-961] and the solution structure of yeast eIF-4E bound to m.sup.7 GDP as determined by NMR spectroscopy [Matsuo H. et al. (1997) Nature Struct Biol.4:717-724] have been described. Both studies describe a cap-binding slot for eIF-4E in which the m.sup.7 G moiety is sandwiched between the side chains of two tryptophans, Trp-56/Trp-102 in mouse and Trp-58/Trp-104 in yeast eIF-4E. A third tryptophan, Trp-166 (both mouse and yeast), as well as Glu-103 in mouse and Glu-105 in yeast, form hydrogen bonds with m.sup.7 G. The cocrystal structure demonstrated additional interactions involving residues Arg-157, Arg-112, and Lys-162 which make direct or water-mediated contacts with the phosphate groups of m.sup.7 GDP. The NMR solution structure of yeast eIF-4E showed that Arg-157, Lys-158 and Glu-159 are close to the phosphate tails of m.sup.7 GDP and M.sup.7 GTP